The present embodiments relate to re-shaping a crucible used in crystal growth.
For positron emission tomography (PET), scintillation crystals are used for detecting positron emissions. Cerium-doped lutetium oxyorthosilicate (LSO:Ce) is a commonly used crystal in PET. Iridium crucibles are used in production for high temperature oxide crystals, such as LSO:Ce, due to several properties of the metal. Iridium will withstand the high temperature (e.g., 2,150° C.) of crystal growth and couples well with radio frequency (RF) power supplies, which are capable of heating the crucibles to such high temperatures.
Cool down from high temperature oxide crystal growth deforms the iridium crucible. Where the crystal material is left to cool inside of the crucible during cool down, a difference in the coefficient of thermal expansion of the iridium and remaining crystal material in the crucible causes the side walls at the bottom of the crucible to swell. This bulge not only stretches the crucible but also thins the metal and ultimately shortens the life of the crucible. If used for more crystal growth, the deformed shape causes a stagnation of melt flow during crystal growth inhibiting successful sequential growth runs. The crucible will eventually swell so large that the crucible can no longer be placed into the ceramics inside the growth chamber. At this point, the crucible is traditionally sent back to the manufacturer to be melted back into sheet and re-fabricated into a new crucible.
Iridium is very difficult and expensive to work, needing diamond tooling, electro discharge machining, and other expensive forming methods to re-fabricate. To prolong use of a given crucible, the crucible is re-shaped a number of times before ultimately needing to re-fabricate due to metal creep and/or grain boundary growth deterioration. To re-shape, the crucible is heated with a tig torch. Once heated, a technician uses a hammer to attempt to remove the warping. This “blacksmith” process may allow for only a few iterations of re-shaping before the crucible needs to be re-fabricated. The “blacksmith” process is also inexact.